ARWadsworth wrote:
"Dave Plowman" wrote in message
...
In article ,
Peter Parry wrote:
Think you'll have problems finding the correct stuff. IIRC, it's
actually 300 ohms.
The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.
snip
Well, if you reckon a car aerial will work ok on all broadcast stations
with the cable replaced with ordinary 50 or 75 ohm cable one, you go
ahead. I'm saying it won't. You could well be right about it being 150
ohm rather than 300 - I wasn't sure hence the IIRC. But I *am* sure it
ain't 75.
It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly,
especially
for MF reception.
Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.
But I think you'll find in practice that neither is an exact match -
receivers have either a manual or auto way of matching the aerial and
cable exactly, and if an extension is introduced this has to be of a
particular capacitance.
So if I was to buy a bog standard car aerial with cable and then cannibalise
this cable for my needs, do you think it will work? Now you mention it I do
remember the car radios where you had to tune into a (was it a weak) MW
station and tweak a pot to match the aerial to the tuner.
Not a pot, a capacitor. ON MW as Peter Parry said, there is considerable
mismatch, and the aerial 'looks' capacitative. The variable cap tunes in
the RF stage to more or less use teh cable resonance to tne the frnt end.
On VHF the antenna is nearly matched as a quarter wave whip, and the
cable will be more or less non-reactive.
--
Adam